This invention relates to methods for making photovoltaic cells and, in particular, to methods for making large area thin film photovoltaic cells with higher yield and greater reliability.
A photovoltaic cell, often referred to as a solar cell, is a semiconductor junction device which converts light energy into electrical energy. A typical photovoltaic cell is a layered structure comprising five principal layers: (1) an absorber-generator, (2) a collector-converter, (3) a transparent electrical contact, (4) an opaque electrical contact and (5) an encapsulant. When light is shone through the transparent contact onto the absorber-generator, the device generates between the two contacts a voltage differential and a current.
The absorber-generator (commonly referred to as the "absorber") is a layer of semiconductor material which absorbs light photons and, as a consequence, generates minority carries. Typically, atoms of the absorber absorb photons and eject electrons thus creating pairs of negatively charged carriers (electrons) and positively charged carriers ("holes"). If the absorber is a p-type semiconductor, the electrons are minority carriers; if it is n-type, the holes are minority carriers. As minority carriers are readily annihilated in the absorber by recombination with the plentiful majority carriers, they must be transported to a region wherein they are majority carriers before they can be utilized to provide power for an electrical circuit.
The collector-converter (the "collector") is a layer of material in electrical contact with the absorber wherein the majority carriers are of the same conductivity type as the minority carriers generated in the absorber. This layer "collects" minority carriers from the absorber and "converts" them into majority carriers. If the collector is an oppositely doped region of the same semiconductor as the absorber, the photovoltaic device is a p-n junction of homojunction device. If the collector is comprised of a different semiconductor, the device is a heterojunction; and, if the collector is metal, the device is a Schottky junction.
The transparent contact is a conductive electrical contact which permits light to pass through to the absorber. It is typically either a continuous transparent film of conductive material or an open grid of opaque conductive material. If the transparent contact is on the same side of the photovoltaic device as the absorber, the device is referred to as being in the front wall configuration. If the transparent contact is on the opposite side, the device is said to be in the back wall configuration.
Although scientists have known about the photovoltaic effect for more than a century, it is only within the past twenty-five years that it could be considered a practical means for generating electricity in useful amounts. Prior to 1950, photovoltaic devices were limited in use to highly specialized applications, such as light metering, wherein conversion efficiency was immaterial and electrical current demand was minimal.
The advent of silicon semiconductor technology in the 1950's permitted the development of high cost, high conversion efficiency silicon junction photovoltaic cells. Arrays of such devices have been used with considerable success in the space program where cost is of little significance. However, the cost of such devices as energy generators, typically from a low of $7,000 per kilowatt to as high as $100,000 per kikowatt, is prohibitively high for terrestrial applications wherein they must compete against conventional generators. While much of this cost is due to the high quality control standards required for spacecraft components, a substantial portion is due to the high cost of preparing silicon crystals of the required purity and due to the inefficiencies of the batch processes by which such cells are fabricated.
While thin film photovoltaic cells possess many potential advantages over silicon cells in terrestrial applications, the fabrication and use of thin film cells has historically been plagued with problems of low yield, non-reproducibility and unreliability. Thin film photovoltaic cells employing thin films of polycrystalline material, such as cadmium sulfide or cadmium zinc sulfide and copper sulfide, offer substantial advantages for the development of continuous processing techniques, and they are flexible and light of weight. Consequently, they offer much promise as solar cells which can be easily fabricated, transported and deployed over large areas. Unfortunately, thin film cells of good efficiency have been difficult to reproduce consistently, and the operating lifetime of the cells produced has been uncertain.
Accordingly, there is a need for a method for making thin film photovoltaic cells with greater yield, reproducibility and reliability.